With this in mind, I found the quote in The Book of Fire between the headings “Confusing The Enemy” and “Three Fierce Shouts”. The text around the quote is:

You must be fully resolved to kill the enemy by any means. You must practice constantly to understand these principles until they become second nature. The warrior attitude is very simple. Focus your mind on your goal, constantly strive to attain perfection, and do not allow yourself to be sidetracked. Think only of winning. You must maintain your own ideals and study properly or you will be sure to lose the Way.

THREE FIERCE SHOUTS

There are only three times when you can scream fiercely in combat: before, during and after. Shouting and screaming are the same as long as they fulfill their purpose, which is to terrify the enemy.

In large-scale strategy we can use your troops to confuse the enemy on the field. Observing the enemy’s spirit, we can make him think, “Here? There? Like that? Like this? Slow? Fast?” Victory is certain when the enemy is caught up in a rhythm that confuses his spirit.

In single combat, we can confuse the enemy by attacking with varied techniques when the chance arises. Feint a thrust or cut, or make the enemy think you are going close to him, and when he is confused you can easily win.

This is the essence of fighting, and you must research it deeply.

The Three Shouts

The three shouts are divided thus: before, during and after. Shout according to the situation. The voice is a thing of life. We shout against fires and so on, against the wind and the waves. The voice shows energy.

Now let us look at the Japanese. Which, interestingly, is not as easy as it sounds because modern Japanese versions of “The Book of Five Rings” are all translations from old Japanese to modern Japanese. And the translations vary greatly! Reading the original text would be like trying to read Chaucer’s Canterbury Tales in the original Middle English. Though to be fair, Japanese has changed much more dramatically than English in the same time period.

The last line in the “To Throw into Confusion” section closely matches Victor Harris’s translation and is roughly “having confused the enemy, winning is easy. This is the essence of fighting research it thoroughly”. Here 能くゝゝ is yokuyoku meaning “exceedingly” and 吟味 is ginmimeaning “careful investigation”. With this, even one not familiar with Japanese can see the Victor Harris work is a translation while Stephen F. Kaufman is an interpretation.

With this evidence then it is clear that the quote “The warrior attitude is very simple. Focus your mind on your goal, constantly strive for perfection and don’t allow yourself to be sidetracked.” should be attributed to Stephen F. Kaufman and not to Miyamoto Musashi.

As there is no original Japanese to find then, the only alternative is to translate the English to Japanese which I offer as follows: